In order to be useful, a coordinate measuring machine must be able to reliably make precision measurements with an accuracy within specfication set by the manufacturer of the machine. Consequently, it is necessary to be able to evaluate the performance of the machine for two reasons. When the machine is initially purchased the purchaser may want to verify that the machine is indeed performing within the quoted specifications before he agrees to accept it. In addition, after the machine has been purchased, in order to ensure that it is operating properly, it is necessary to make periodic performance measurements to ensure that the machine is operating within acceptable tolerances.
There are various ways to perform tests which can evaluate the accuracy of a coordinate measuring machine. However, one set of standard tests has been developed by the American Society of Mechanical Engineers (ASME) for evaluating the performance of coordinate measuring machines. These tests are well-known in the art and are described in detail in a proposed ASME standard, No. B89.1.12, issued on Oct. 1, 1983.
The ASME standard prescribes various test procedures, for testing performance criteria such as repeatability, linear displacement accuracy and volumetric accuracy. The method specified in the ASME standard for checking volumetric accuracy limits uses a ball bar gauge.
As is well-known in the art, a ball bar gauge is a rigid bar with a precision diameter tooling ball affixed to each end. The ball bar is used with two sockets which are mounted on the machine to be tested. In accordance with well-known measurement technique, these sockets each contain magnetic chucks which attract and hold one of the tooling balls on the end of the ball bar gauge. To test a coordinate measuring machine with the ball bar gauge, one magnetic socket is mounted on the workpiece supporting surface and the other socket is mounted on the mechanism which normally holds the measuring probe. One of the balls at each end of the ball bar is placed in one of the sockets. Ihe machine and ball bar is moved to various angular positions specified in the ASME standard and the machine read-outs observed. The variation in the readings indicates the working tolerance for the machine.
In order to achieve a proper measurement with the above method, it is necesssary to utilize a ball bar gauge with balls that are spherical within very close tolerances. Tooling balls with the required tolerances are readily available commercially, but it has been a problem in the prior art to attach the commercial balls to the gauge bar while maintaining the necessary tolerances.
Several prior art methods have been utilized to provide this attachment. In one method a tapped hole is drilled in the ball for receiving a threaded stud which can then be screwed into a tapped hole in the bar. However, the drilling and tapping operations generally distort the ball so that the required tolerances cannot be achieved.
Another prior art attachment scheme has been to cement the balls to the ends of the bar with a suitable epoxy cement. However, a gauge with such attachments is fragile and susceptible to damage by dropping. Furthermore the balls cannot be easily replaced when damaged and thus the entire bar must be discarded if either ball is damaged.
In yet another prior art arrangement the tooling balls are machined with integral studs or projections which can then be fastened to the rigid bar by means of set screws. One problem with this arrangement is that machine balls with such integral studs cannot meet the tolerances which is typically available for balls which are completely spherical.
It is therefore an object of the present invention to provide a ball bar gauge in which the tooling balls can be spherical tooling balls which are commerically available.
It is another object of the present invention to provide a ball bar gauge which is physically rugged and not subject to damage from impacts.
It is yet another object of the present invention to provide a ball bar gauge in which the balls can be easily removed for replacement.
It is yet a further object of the present invention to provide a ball bar gauge which has a simple construction which can be readily assembled using simple parts.